The increased military and civilian use of helicopters has resulted in a need for improved safety devices to insure the safety of occupants during crash conditions. Occupant protection and survival under crash conditions is of primary importance in helicopter design. Adequate protection requires that crash forces and decelerations transmitted to the occupant through the helicopter structure are limited to within human tolerance levels. When vertical loads from a crash impact exceed human injury tolerance, severe back injury can occur. Seats have been designed with various different forms of integral energy absorbing structure which allows the occupant of a seat to stroke downwards in a controlled manner thereby reducing the magnitude of deceleration that is experienced, while increasing its duration.
Conventionally, aircraft seats have been designed for mounting on a floor, a bulkhead, or directly on aircraft structure. Floor warpage or bulkhead buckling during a crash landing can adversely affect the correct operation of the seat.
Energy attenuating seats, also referred to as energy absorbing seats, have been proposed in which the seat limits the vertical crash loads experienced by the occupant to within human injury tolerance levels. The vertical crash loads are limited by allowing the seat motion or stroke to be load limited by an energy attenuator or energy absorbing device. Existing energy attenuating seats require a cleared volume from underneath the entire stroking seat pan where the seat will move during the stroke. The area under the seat pan must not contain any rigid structure or other obstructions that could interfere with the seat motion in a crash. The seat bottom to seat back relative angle remains basically constant during the stroking. Therefore a large volume of otherwise usable space under the seat must be dedicated and preserved to use present energy attenuating seats. Furthermore, such seats cannot be used where the seat bottom is mounted directly on the vehicle structure.
A need has thus arisen for an energy attenuating seat which will provide energy attenuation where the seat is mounted directly on the aircraft structure or where an entire cleared space under the seat is not practical. A need has further arisen for an energy attenuating seat which reduces the potential for occupant submarining out from under a lap seat belt, and which further provides for vertical seat adjustment.